An ultra thin flat panel display device has a display screen with a thickness of several centimeters. Especially, a liquid crystal display (LCD) device among the flat panel display device is widely used for monitors of notebook computers, spacecrafts, and aircrafts, owing to features and advantages of low driving voltage, low power consumption, and portable size.
The LCD device includes a lower substrate, an upper substrate, and a liquid crystal layer formed between the substrates. Generally, a thin film transistor and a pixel electrode are formed on the lower substrate, and a light-shielding layer, a color filter layer and a common electrode are formed on the upper substrate.
As mentioned above, the LCD device includes various elements, and a number of processes are repeatedly required to form the elements. Therefore, to improve productivity under the mass production system, various efforts are required in the process of forming the elements constituting the LCD device. Examples of efforts include reducing the process time, improving process devices to reduce the manufacturing cost, and developing a new process. Therefore, various improvements have been suggested.
As an example of such improvements, there is an improvement in the process of making a thin film transistor formed on a lower substrate of an LCD device. Hereinafter, the process of making a thin film transistor will be described in more detail with reference to the accompanying drawings.
FIG. 1 is a sectional view illustrating a thin film transistor formed on a lower substrate for a general LCD device.
As shown in FIG. 1, a gate electrode 12 is formed on a substrate 10, and a gate insulating film 14 is formed on a gate electrode. A semiconductor layer 16 is formed on the gate insulating film 14, and a source electrode 18a and a drain electrode 18b are formed separated from each other on the semiconductor layer 16.
To form such a thin film transistor in the related art, a mask for patterning the gate electrode 12, a mask for patterning the semiconductor layer 16, and a mask for patterning the source and drain electrodes 18a and 18b were required. In other words, in the related art, three masks were required to form the thin film transistor, and three pattern formation processes were required because the elements were separately patterned.
In this respect, studies for reducing the number of pattern formation processes have been performed. As a result, a method for patterning the semiconductor layer 16 and the source and drain electrodes 18a and 18b using one mask through diffraction exposure has been recently developed.
FIG. 2A to FIG. 2D are sectional views illustrating a process of forming a thin film transistor to reduce the number of masks and pattern formation processes.
As shown in FIG. 2A, a gate electrode 12 is formed on a substrate 10, and a gate insulating film 14, a semiconductor layer 16 and a metal layer 18 for source and drain electrodes are sequentially formed on the gate electrode 12.
Afterwards, as shown in FIG. 2B, a mask pattern 20 having a step difference is formed on the metal layer 18 using diffraction exposure. A method of forming the mask pattern 20 having a step difference using the diffraction exposure is shown in FIG. 3A to FIG. 3C.
First, as shown in FIG. 3A, a resist layer 20 to be a mask pattern is formed on the substrate 10. Then, as shown in FIG. 3B, a diffraction mask 30 is disposed on the substrate 10 where the resist layer 20 is formed, and then light is irradiated thereon. The diffraction mask 30 includes a light-transmitting region 30a, a light-shielding region 30b, and a partially light-transmitting region 30c. Thereafter, the mask pattern 20 having a step difference is completed by a developing process as shown in FIG. 3C. During the developing process, the resist layer corresponding to the light-transmitting region is removed, the resist layer corresponding to the light-shielding region remains, and the resist layer corresponding to the partially light-transmitting region is partially removed. Thus, the mask pattern 20 is formed with a step difference.
Afterwards, as shown in FIG. 2C, the semiconductor layer 16 and the metal layer 18 at left and right sides of the mask pattern are removed by an etching process.
Subsequently, as shown in FIG. 2D, the metal layer 18 at a middle portion of the mask pattern is removed to form the source and drain electrodes 18a and 18b. The mask pattern 20 is finally removed to complete the thin film transistor.
As described above, the semiconductor layer and the source and drain electrodes are formed by one process using the mask pattern 20 manufactured with a step difference by the diffraction exposure, thereby improving productivity.
The above method based on one mask has advantages in that the process steps were simplified and the process time was reduced in comparison with the related art method for forming a semiconductor layer and source and drain electrodes using two masks. However, in the method based on one mask, there exists a problem in that the process time is still long because exposure and developing processes are required. Moreover, a problem occurs in that the manufacturing cost increases because a diffraction mask of high cost is required.